Now that we are all set up and ready to tune, where do we start?  Well, I spent a few days looking around on some of the many Subaru enthusiast web forums, paying attention to their engine management discussions.  Of most use to me was a forum dedicated especially to StreetTuner on iwsti.com.  In this forum, I found some folks who had already started down this path.  In particular, some folks were finding that their cars were reaching loads much higher than the highest loads mapped in the basemaps supplied by Cobb.  Since StreetTuner allows you to change the column values, they rescaled the maps and made them available, otherwise unchanged, to the community.  I used one of these rescaled “Stage2” base maps as my starting point.  

The first thing I did from there was to change the Ignition Advance Multiplier initial value to 1.  Normally, this is at 0.5.  What this value does is specify the IAM value the ECM will utilize directly after a reset.  It does not prevent the value from changing, should the ECM want to change it.  The function of the IAM value is to tell the ECM how much of the Dynamic Advance maps should be used.  If the value is 0.5, half the values in the advance maps will be used.  If the value is 1, the full values will be used.  Now, leaving this at 0.5 will only mean you have to wait a few drive cycles for the ECM to decide to raise it all the way.  This means you won’t see the full results of your tuning until after it has done so.  I don’t like waiting.

The next step I took in this process was to attempt to get the fuel maps working as they should with respect to my intake.  As I mentioned earlier, I have an aftermarket high flow intake.  One of the biggest problems with most intakes on the market is that they alter the shape and flow characteristics of the MAF sensor so that a given amount of air flowing into the engine is incorrectly translated by the ECM and thus, the fueling will be unstable and inaccurate.  To prevent this behavior, you need to calibrate the flow vs voltage characteristics of the MAF sensor.  The Intake Calibration table is where you perform this adjustment.  Now, the actual process of adjusting this table can be daunting.  The way Cobb and others have recommended doing it was to look at the live data logging feature (Dashboard) and view the Air Fuel Trim Learned and Air Fuel Correction values.  The idea is to adjust each MAF voltage (presumably with Live Trace helping you follow it) so that the sum of the Learning and Correction values is 0.  I tried this.  It was impossible and frustrating.  You could make a change and watch the values start to drift towards 0 and then wham, overshoot and pretty soon you’re completely wrong again.  That’s obviously not working.  I came up with a better idea.  Log.  A lot.  You want to log the MAF Voltage, MAF Airflow, and Air/Fuel Learning values.  That’s really all you need.  Make sure you’ve been driving the car for long enough so that the Learning values are all about where they are gonna stay, then go do a 10-20 minute drive and log the whole thing.  When you’re done, you’ll have a huge set of data that you need to put into Excel for manipulation.  First, sort the entire dataset by MAF voltage.  You’ll see that you have plenty of repeats.  The idea now is to average all the Learning values for a given MAF voltage and use that as your correction percent which you’ll add to the Airflow.  This can be done fairly easily with Excel using an array function. 

For more information on this process, look for my posting on iwsti.com on the topic.  Once this was done, I could now start tuning my fuel, comfortable that my air fuel trims would not be altering my actual running air fuel ratios a week from now.